For forming semiconductor devices like CMOS devices (Complementary Metal Oxide Semiconductor), MOSFET devices (Metal Oxide Semiconductor Field Effect Transistor) or high memory devices such as DRAMs (Dynamic Random Access Memories), it is often useful to form a thin, high dielectric constant (high-k) film onto a substrate, such as a silicon wafer. A variety of techniques have been developed to form such thin films on a semiconductor wafer.
In the past, gate dielectric layers have been formed using silicon dioxide. The scaling down of the above described devices, however, has increased the demand for gate dielectrics with a higher dielectric constant than silicon dioxide. This is necessary to reach ultra thin oxide equivalent thicknesses (EOT, Equivalent Oxide Thickness) without compromising gate leakage current.
In detail, as semiconductor devices have scaled to smaller dimensions, effective gate dielectric thickness has gotten thinner. The continued scaling of conventional gate dielectrics, such as SiO2 and SiOxNy, has almost reached the fundamental limits of very high gate leakage current, due to direct tunneling, which is not acceptable in a scaled device requirement of a low leakage current. In order to suppress the high leakage current, several high-k films of transition metal oxide and silicate, such as HfO2, ZrO2, Hf-aluminate, Zr-aluminate, Zr-silicate, Hf-silicate and a lanthanide oxide like La2O3, Pr2O3, and Gd2O3, have been studied in replacement of SiO2 and SiOxNy.
However, these conventional materials have shown a number of disadvantages. According to S. OHMI, et al., “Rare earth metal oxide gate thin films prepared by E-beam deposition”, International Workshop on Gate Insulator 2001, Tokyo, Japan, it is known that ZrO2 or HfO2 has shown micro crystal formation, resulting in high leakage current.
Furthermore, from J. H. LEE, et al., “Poly-Si gate CMOSFETs with HfO2—Al2O3 laminate gate dielectric for low power applications”, Tech. Dig. VLSI, page 84, 2002, it is known that HfO2—Al2O3 laminate or Hf-aluminate have serious mobility degradation due to fixed charges in the high-k dielectric film.
Moreover, TAKESHI, YAMAGUCHI, et al., “Additional scattering effects for mobility degradation in Hf-silicate gate MISFETs” Tech. Dig. IEDM 2002 reports that in case of Zr-silicate or Hf-silicate, phase separation of the film into HfO2 and SiO2 regions by the high temperature anneal induces also mobility degradation.
For lanthanide oxides, leakage current results have indicated that lanthanide oxides may be possible candidates of future dielectrics. However, according to H. IWAI, et al., “Advanced gate dielectric materials for Sub-100 nm CMOS”, Tech. Dig. IEDM 2002, it is reported that these lanthanide oxides also form interfacial layers on Si substrates after subsequent thermal annealing, which may indicate thermal instability of these lanthanide oxides.
Moreover, impurity penetration such as boron penetration from e.g. a gate layer to a Si substrate is a further problem to be solved by these high-k dielectric films. Even though nitrogen incorporation on HfSixOy has been known to suppress impurity penetration (e.g. boron penetration) and improve thermal stability, it was also reported that a very high Si content of Si/[Si+Hf] ratio of over 80% in HfSixOyNz prevents flat band voltage shift, resulting in seriously reducing dielectric constant of the film even with a high nitrogen content of 30 atomic percent (see M. KOYAMA, et al. “Effects of nitrogen in HfSiON gate dielectric on the electrical and thermal characteristics”, Tech. Dig. IEDM 2002, 34-1). This high Si content low-k dielectric film of HfSixOyNz is almost the same as conventional SiOxNy in terms of dielectric constant. Other technical problems of the formation of respective films on Si substrate are that a high nitrogen concentration at the interface between the dielectric layer and the Si substrate can be induced by subsequent high thermal annealing to degrade mobility.
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